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TR2/3/3A gas gauge or sender or ???

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Sumton

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57 tr3. been restoring it for a few years, got it running just a few months ago. been working through problems as the stupid mechanic who supposedly put most of this together before abandoning this project "because i wasn't happy". gas gauge was supposedly rebuilt etc a number of years ago. sending unit is relatively new; maybe 6 months? replaced the sending unit because the old one, also supposedly new a number of years ago, did not register anything at all to the gauge (as far as i can tell, never worked). how do i test the sending unit?

gas gauge reads dead empty until something over 1/2. then it jumps to 1/2. as soon as you start driving, it varies (with 1/2+ in the tank) between 1/2 and full, and skitters a bit. all other gauges seem to work properly.
 
No, that is backwards. The TR3 gauge works differently than the later cars; reads less full for lower resistance. Sorry I don't recall the range offhand, but I know open circuit is past full.

But you're on the right track. Check the sender resistance as you move the arm; it should change smoothly and more or less evenly from empty to full. You can use a yardstick through the filler to push the float down, or a bent coat hanger to pull it up.

Once you find out what the correct range is (or just accept what you have), you can use the techniques outlined at https://mgaguru.com/mgtech/electric/fg_06.htm to test the gauge. As Barney shows on that site, there are two nuts on the back that allow for adjustment, but they are tough to get right. I found that just being mounted in the metal panel made a difference in the reading as well.

Another tip is to add a ground wire for the gauge. The factory ground through the mounting clamp is far from secure. Just a simple jumper from the mounting stud to the nearby ground tie point on the instrument panel will do. I also added a ground wire for the tank sender, since otherwise the tank only grounds through the fuel line.

You can see my fuel gauge ground wire in this photo (taken as I was parting out my previous TR3A after it got wrecked).
 
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Hopefully an adjustment is what you need but when my gauge was erratic and only stopped at some points on the scale I used the website Randall linked to and found one very thin wire broken inside the unit. I carefully repaired that break and all has been well since. I do not recall which wire it was but I do recall it seemed about as fine a human hair.
 
given the erratic nature of the gauge, i'm going to try grounding it first. hopefully that will solve the problem. if that doesn't work, mgaguru is quite a resource! thank you for all the info above.
 
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